Being a byproduct of primary photosynthesis stages, Chl-a fluorescence
emission depends to a large extent on the physiological state of
photosynthetic apparatus and, in particular, on the state of PS II RCs. If the
photosynthetic apparatus of PS II is in good functional state, under weak
light the majority of PS II RCs are in open state (P680 Pheo Qa, where P680,
Pheo and Qa are respectively the primary donor, the intermediate pheophytin
acceptor, and the first quinone acceptor in PS II RC), providing maximum
efficiency of light energy conversion.
In this case Chl-a fluorescence yield is minimal (F ). The influence of
min
saturated light (or addition of herbicide DCMU) leads to transfer of RCs to
the so-called closed state with reduced Qa ( P680PheoQK ). The resulting
fluorescence yield reaches its maximum value $> due to addition of variable
m
fluorescent component originating from charge recombination in the closed PS
II RCs. In common case, the actual fluorescence yield (
minimum and maximum values, depending on the states of PS II RCs. The
difference $ = $ - $> is called variable fluorescence.
V Ifl
3. - LIDAR IMPLEMENTATION OF PUMP-AND-PROBE TECHNIQUE
The basic idea of pump-and-probe technique (Falkowski and Kiefer, 1985)
is the use of special actinic (pump) flash of light which is sufficiently
intense to cause the transition of all initially open PS II RCs to the closed
state (P680PheoQA ). The maximum Chl-a fluorescence $ is measured in response
m
to excitation by the weak probe flash of light, following the pump flash with
a certain delay. The actual Chl-a fluorescence $> is measured in response to
the probe flash preceding the pump flash.
Relying on these measurements, corresponding relative increase in Chl-a
fluorescence yield (i.e. the relative yield of variable fluorescence) n = (i>
m
- $)/ = Ai>/ can be calculated. The possibility to estimate this parameter
n> m
with pump-and-probe technique is of particular importance, as its value is a
quantitative measure of the efficiency of light energy conversion in
photochemical reactions in PS II (e.g. see Falkowski and Kiefer, 1985).
Recently it has been shown (Genty et al., 1989) that this parameter determines
the quantum yield of non-cyclic electron transport due to photochemistry.
Since the pump-and-probe technique allows to conduct relatively simple
measurements of in a wide range of variations of background irradiance,
the use of this technique provides the direct way to monitor the on-going
photosynthesis under ambient light in natural conditions.
The implementation of this approach has conceptually increased the
potential of fluorometric techniques in the field, in particular - in sea
research (Falkowski and Kolber, 1990; Falkowski et al., 1991). The use of
submersible pump-and-probe fluorometer provides information about vertical
profiles of the on-going photosynthesis in real-time, in situ, nondestruc
tive^. Recent field studies indicated a good agreement between these
estimates and conventional radiocarbon-based measurements of primary
production (Falkowski and Kolber, 1990; Falkowski et al., 1991). Nevertheless,
as in the case of any conventional shipboard observations, the common problems
are relatively low space/time resolution and corresponding difficulties in
investigations of fast-developing processes distributed over large areas.
The next essential step is therefore the development of lidar implementa
tion of pump-and-probe technique, capable of remote monitoring both photosyn
thetic efficiency and phytoplankton abundance in the sea from onboard a moving
carrier (ship, aircraft, helicopter). Our idea (Chekalyuk and Gorbunov, 1992)
was to use pulsed lasers as sources of pump and probe light flashes in the
water column. Due to low beam divergence and high power of laser pulses, in
principle it is possible to ensure required modes of excitation remotely.
